1. Field of the Invention
The present invention relates to an ultrathin-walled rolling bearing used in industrial robots, machine tools, medical instruments, etc.
2. Prior Art
FIG. 6 shows an example of a CT scanner, which is a kind of medical instrument. As shown, the CT scanner device is so designed that X-rays generated by an X-ray tube assembly 50 are radiated to a subject 53 through a wedge filter 51 for uniformizing intensity distribution and through a slit 52 for restricting intensity distribution. The X-rays passing through the subject 53 is received by a detector 54, converted into electric signals, which are sent to an unillustrated computer. Such parts as the X-ray tube assembly 50, wedge filter 51, slit 52, and detector 54 are mounted on a substantially cylindrical rotatable block 57 rotatably supported through a rolling bearing 55, and the rotary driving of said rotatable block 57 causes rotation of the parts around the subject 53. In the CT scanner device, the rotary movement of the X-ray tube assembly 50. and detector 54 opposed thereto around the subject 53 provides projection data covering all angles at every point in the inspection cross-section of the subject 53, and a tomogram is obtained from these data through a reconstructive program prepared in advance.
In this CT scanner device, the inner peripheral surface of a stationary block 56 disposed inside the rotatable block 57 is formed to have a sufficiently large diameter (about 1 m in diameter). to receive the subject 53, so that a so-called ultrathin-walled roller bearing, which is very small in cross-section for its diameter, is used as the bearing 55 disposed between the stationary block 56 and the rotatable block 57.
Heretofore, the four-point contact ball bearing, which is capable of supporting a high-moment load acting in opposite directions by using a single bearing and which saves space, has been mainly used as the rolling bearing 55 particularly of the low speed rotation type (about 60-90 rpm) for this CT scanner device. Conventionally, for the sake of assembly, the internal clearances for four-point contact ball bearings are set at 0 or above, of which clearances, the radial internal clearance, for example, is set often in the range of 0.005-0.050 mm in consideration of machining error, etc.
In recent CT scanner devices, further reduction of vibration and improvements in acoustic characteristics are desired from the standpoints of lightening patient""s burden, improving radiographic accuracy, etc. With the conventional construction, however, these desires cannot be fully met and further improvements are desired.
Accordingly, an object of the invention is to achieve reduction of vibration and improvements in acoustic characteristics of ultrathin-walled rolling bearings, at low cost.
To achieve said object, the invention provides an ultrathin-walled rolling bearing including an outer member having a raceway surface in the inner periphery, an inner member having a raceway surface in the outer periphery, a plurality of rolling elements interposed between the raceway surfaces of the outer and inner members, and a cage for holding the rolling elements in circumferentially equispaced relationship, wherein the ratio of the diameter of the rolling elements to the pitch circle diameter of the bearing is not more than 0.03, the rolling elements are arranged in a single row, being in two-point contact with each of the raceway surfaces of the outer and, inner members, and the internal clearances of the bearing are made negative.
This bearing is a so-called four-point contact rolling bearing, and in this type of bearings, when the internal clearances are made negative, play between the rolling elements disappears. Therefore, the bearing rigidity improves and reduction of noise and vibration due to play between the balls is achieved.
In this case, if one of the inner and outer members is fixed to the rotatable block of a CT scanner device rotating around a subject and the other to the stationary block thereof, then reduction of patient"" burden which is brought about by reduction of noise and vibration of said rolling bearing, and improved radiographic accuracy can be achieved.
By constituting one of the inner and outer members by first and second raceway members opposed to each other on the opposite sides of the axial clearance, and adjusting the width of the axial clearance, the internal clearances of the bearing can be made negative by simple means and at low cost. The adjustment of the width of the axial clearance is made by axially relatively moving the first and second raceway members.
Further, by eliminating the difference in the amount of thermal expansion after the rolling element have been installed between the inner and outer members which differ in the amount of thermal expansion, the internal clearances of the bearing can also be made negative by simple means and at low cost.